Small coils are being used as antennas for near field communications (NFC). For example, mobile phones are incorporating contactless smartcard circuits to read tags and communicate with readers in mobile payment applications.
FIG. 1 shows a prior art mobile device 100 with a small coil 110 coupling to a large coil 120 in a reader device. FIG. 2 shows a prior art transformer representation of the coils of FIG. 1. When in the near field, coils 110 and 120 couple as transformer windings as shown in FIG. 2. Because coil 110 is small, it presents a small coupling area as compared to reader coil 120, and transformer coupling is not very efficient. As a result, the mobile device generally needs to be placed very close to a reader device in order to effect communications.
FIG. 3 shows a prior art back-to-back transformer. Back-to-back transformers are typically used for line isolation. See Darren Ashbey et al., Circuit Design:Know It All 199-200 (2008).